In recent years, the number of users of mobile communications including portable telephones has grown remarkably, presenting a problem of how to effectively use frequencies of radio waves used for transmission and reception. Techniques for the effective use of the frequencies include reduction of the radius of each cell having a base station at its center, antenna sectorization and the like. At present, sectored antennas currently used at the base station each has a fixed antenna pattern. If the antenna pattern of each of the sectored antennas can be adaptively varied, an optimum beam can be formed in accordance with traffic which varies momentarily, so that the effective use of the frequencies becomes feasible.
To adaptively vary the antenna pattern, several pattern synthesis techniques utilizing a circular array antenna (hereinafter sometimes referred to as “circular array”) are proposed. For example, the paper entitled “Pattern Synthesis of Circular Arrays with Many Directive Elements” by F. I. Tseng and D. K. Cheng, published in the November 1968 issue of the IEEE Transactions on Antennas and Propagation, vol. AP-16, No. 11, pp. 758–759, discloses a calculating method for transforming excitation coefficients for a linear array antenna (hereinafter sometimes referred to as “linear array”) having an odd number of elements into excitation coefficients for a circular array having the same number of elements as the linear array antenna. The method disclosed in this paper, however, is limited to cases where an array antenna has an odd number of elements, not referring to cases where it has an even number of elements.
Another paper entitled “An Adaptive Zone Configuration System using Array Antennas” by Kazuo Kubota, Tsukasa Iwama and Mitsuo Yokoyama, published in the September 1995 issue of Technical Report of IEICE, RCS59–76, discloses a method, utilizing the method described in the above-mentioned paper, for transforming excitation coefficients for a linear array antenna having an odd number of elements into excitation coefficients for a circular array having an even number of elements (one element fewer than the linear array antenna). However, according to this paper, a controlled antenna pattern does not reflect a desired beam direction and a desired beam width, and consequently, a desired antenna pattern cannot be obtained.